Restoring natural forests is the best way to eliminate atmospheric carbon

Maintain global warming below 1.5 ° C to avoid dangerous climate change¹ requires the elimination of vast amounts of carbon dioxide from the atmosphere, as well as drastic reductions in emissions. The Intergovernmental Panel on Climate Change (IPCC) estimates that about 730 billion tonnes of CO₂ (730 petagrams of CO₂, or 199 petagrams of carbon, Pg C) must be removed from the atmosphere by the end of the century². This is equivalent to all the CO₂ issued by the United States, the United Kingdom, Germany and China since the industrial revolution. Nobody knows how to capture so much CO₂.

Forests must play a role. Locking carbon in ecosystems is proven, safe and often affordable³. The increase in tree cover has other benefits, ranging from protecting biodiversity to managing water and creating jobs.

According to the IPCC, increasing the total area of ​​the world's forests, woodlands and savannahs could store about a quarter of the atmospheric carbon needed to limit global warming to 1.5 ° C above preindustrial.². In the short term, this means adding 24 million hectares of forest each year by 2030.

Decision makers are sowing the seeds. For example, in 2011, the German government and the International Union for Conservation of Nature launched the Bonn Challenge, which aims to restore 350 Mha of forest by 2030. As part of this initiative and in other countries, 43 countries in the tropics and subtropics where fast growing trees, including Brazil, India and China, have committed nearly 300 Mha of degraded land (see additional information, Table S1). . It's encouraging.

But will this policy work? We show here that in current plans, this will not be the case. Closer examination of country reports reveals that almost half of the promised acreage will become commercial tree plantations (see Table S1). Although these can support local economies, plantations store much less carbon than natural forests, which grow with little or no man-made disturbance. Regular harvesting and clearing of plantations releases stored CO₂ return to the atmosphere every 10-20 years. On the other hand, natural forests continue to sequester carbon for several decades⁴.

To stop global warming, deforestation must stop. And restoration programs around the world should put all degraded lands back into natural forests – and protect them. More carbon needs to be stored on land, while taking into account competitive pressures for the provision of food, fuel, fodder and fiber.

We call on the restoration community, forestry experts and policy makers to prioritize the regeneration of natural forests over other types of tree planting, allowing disturbed lands to return to their previous state of carbon. . This will involve tightening definitions, transparently reporting plans and results, and clearly indicating trade-offs between different land uses.

Misdirected efforts

To combat climate change, the most effective place to plant trees is in the tropics and subtropics – this is where most of the forest restoration commitments are. Trees grow and absorb carbon fast near the equator and land is relatively inexpensive and available (see go.nature.com/2ogmbmz and "restoration potential"). Forest establishment has little effect on the albedo (reflectivity) of the soil surface, unlike high latitudes, where trees mask snow that would otherwise reflect solar energy and help cool the planet. Well-managed forests can also help reduce poverty in low-income areas, preserve biodiversity and support the United Nations Sustainable Development Goals – including Goals 1 (no poverty), 6 (safe water), 11 (sustainable communities). , 13 (climate action) and 15 (life on earth).

So far, just over half (24) of the countries participating in the Bonn Challenge and other programs have published detailed restoration plans covering two-thirds of the total area announced (Table S1 ). Nations follow three main approaches. First, degraded and abandoned farmland will remain in the natural forest. Second, marginal farmland must be converted into valuable tree plantations, such as Eucalyptus for paper or Hevea braziliensis for rubber. Third, agroforestry involves growing useful crops and trees.

Natural regeneration is the cheapest and technically simplest option. Just over one-third (34%) of the total area allocated should be managed in this way. Land protection from fire and other human disturbances allows trees to return and forests to thrive, creating carbon stocks that can quickly reach the level of a mature forest in about 70 years.⁴. Planting native species can accelerate recovery times and the area under natural regeneration is expanded through legislation and incentives such as those developed in Costa Rica.

However, plantations are the most popular restoration plan: 45% of all engagements involve planting large monocultures of trees as profitable businesses. The majority of them are expected in large countries such as Brazil, China, Indonesia, Nigeria and the Democratic Republic of Congo (Table S1). Brazil, for example, has committed 19 Mha to wood, fiber and other plantations, more than double its current 7.7 Mha.

Agroforestry represents the rest (21%). This practice is widely used by subsistence farmers, but rarely on a large scale. Some crops benefit from trees, such as coffee grown in the shade or corn intermingled with trees that improve nitrogen in soils. The trees themselves provide fuel, lumber, fruit or nuts.

As a result, two-thirds of the world's reforestation area for carbon storage is for crop production. This raises serious concerns.

First, plantations contain little more carbon, on average, than land cleared for planting. Clearance releases carbon, followed by rapid uptake of fast-growing trees such as Eucalyptus and Acacia (up to 5 tonnes of carbon per hectare per year). But after harvesting these trees and clearing land for replanting – usually once a decade – carbon is released through the decomposition of waste and plantation products (mainly paper and wood chips).

It may be possible to increase the amount of carbon stored in plantations by using them less often, using different species or by turning wood into more sustainable products.³. But little field research has been done, in part because it could reduce plantation yields.

Secondly, a considerable increase in the area of ​​plantations could undermine their profitability, which is why countries define them in order of priority. If current restoration plans are adopted, the number of global and tropical plantations will increase by 157 Mha to 237 Mha. This would mark a major change in land use globally⁵. Prices for wood chips and paper products would likely fall. But there has been no research on the potential economic effects of this major change in forest policy⁵.

Third, policymakers misunderstand the term "forest restoration". For example, few conservationists believe that this should include planting a monoculture of Eucalyptus trees for the regular harvest. But by using broad definitions and confusing terminology, policy makers and their advisors mislead the public.

It is true that many plantations meet the definition of forest defined by the United Nations Food and Agriculture Organization: more than 0.5 hectare of area, trees with a height of at least 5 meters and a vegetal cover higher than 10%.⁵. Yet key components of climate change mitigation and biodiversity protection are lacking. Plantations are important from an economic point of view, but they should not be classified as forest restoration. This definition needs to be urgently revised to exclude monocultures.

Fourth, reports often confuse the regeneration process in natural forest with the type of land cover resulting⁶. Land can be described as natural forest when it is far from ripe. Meanwhile, calculations of climate benefits usually assume that these lands become forests and remain so forever. But there is no guarantee that these forests will be protected in 50 or 100 years, especially as the demand for land increases.

Four paths

Restoring natural forests is clearly the most effective approach to storing carbon. But conflicting priorities sabotage the potential for carbon storage. As an illustration, we calculated carbon uptake as part of a series of four restoration scenarios pledged by 43 countries as part of the Bonn Challenge and national programs (see "What strategy?"). Additional methods "). In the first scenario, today's commitments are extended to 2100. In the second, they are kept until 2050, after which the natural forest is converted into biofuel plantations. In the third, the entire area (350 Mha) regenerates into natural forest. And in the fourth, everything becomes plantations.

In short, if all 350 Mha is devoted to natural forests, they will store an additional 42 Pg C by 2100 (see "What strategy?"). Giving the same area exclusively to plantations would sequester only 1 Pg C or, if it is only used for agroforestry, 7 Pg C. In addition, on average, natural forests are 6 times better than agroforestry. and 40 times better than plantations. carbon (sequestration of 12, 1.9 and 0.3 Pg C per 100 Mha by 2100, see Additional information, Table S6).

Thus, in current patterns, natural forests can help us achieve the average estimate of forest uptake used in IPCC pathways to maintain warming to less than 1.5 ° C, 57 Pg C (Our storage value is lower due to more optimistic estimates of tree growth and area planted in some model plots.²Any other approach will be far from satisfactory.

Maintaining the currently reported combination of natural forest, plantation and agroforestry restoration allows sequestration of about one third of the carbon (16 Pg C) of the natural forest scenario only, mainly due to the inefficiency of the plantations (see "What strategy?"). Even that might be optimistic, as it assumes that all new forests are protected, while climate policy itself may threaten them.

Another technology to remove carbon from the atmosphere is at the heart of the 1.5 ° C temperature pathway: Bioenergy with Carbon Capture and Storage (BECCS). This should eliminate 130 pg from here 2100². This technology would require an additional 380-700 Mha by mid-century to grow biofuels⁷. Eucalyptus, maize and switchgrass (Panicum virgatum) would be burned in power plants and carbon emissions captured and stored underground. This huge additional demand for land could displace restored forests. We estimate that the conversion of planned new natural forests to bioenergy crops after 2050 significantly reduces sequestration to 3 Pg C by 2100. This also delays by several decades the time when BECCS becomes carbon negative.⁷.

Our estimates have many uncertainties, ranging from the exact location where the restoration will take place, to the species that can be planted and to their carbon sequestration rates. Future CO₂ Fertilization and climatic impacts on future forests are particularly uncertain and could be better assessed using terrestrial system model tests (see Additional Information).

Critics will argue against the unrealistic idea of ​​expecting all natural forests to be protected in perpetuity. Certainly, agriculture based on trees and plantations are essential elements of many landscapes. What is needed is an extension of the restoration program, not a retreat. It is possible to reach 350 Mha of new natural forest as part of a much larger total area that would include plantations and agroforestry.

As the Earth's land area is limited, there is a need for research to find optimal solutions to competing pressures on the world's land, whether for food, fuel, fodder, fiber, or ecosystem services.⁸. However, these pressures do not go in one direction: it is possible that agricultural productivity increases to free up land, as well as changes in consumption habits, especially towards diets low in meat and dairy products. And synergies exist: using habitat restoration to link existing forests would allow species to move as the climate changes, thus reducing future extinction.

And then?

Current forest restoration programs need to increase their carbon sequestration potential to meet global climate commitments. We suggest four ways for this to happen.

First and foremost, countries should increase the proportion of land being regenerated to become natural forests. Each additional 8.6 Mha sequesters 1 extra Pg C by 2100. This is an area about the size of the island of Ireland, or the state of South Carolina.

Secondly, give priority to natural regeneration in the humid tropics, such as the Amazon, Borneo or Congo Basin, which all have very high biomass forests compared to drier regions. International payments to recreate and sustain new forests from carbon sequestration, climate adaptation or conservation funds could mobilize action.

Third, build on existing carbon stocks. Target degraded forests and partially forested areas for natural regeneration; concentrate plantations and agroforestry systems on treeless areas and, as far as possible, choose agroforestry rather than plantations.

Fourth, once the natural forest is restored, protect it. This could be by expanding protected areas; give property rights to indigenous peoples who protect forest lands; change the legal definition of how land can be used so that it can not be converted to agriculture, or encourage commodity companies to commit to not clearing land. restored natural forests.

The ambitious global restoration agenda is good news. And last month's declaration that the 2020s will be the United Nations Decade for the Restoration of Ecosystems affirms its importance. But these efforts will only remove enough carbon from the atmosphere if the restoration of the forest is considered as such: the permanent restoration of largely natural and largely intact forests.